There is a significant quality control problem associated with two classes of surface imperfections known as pits and mounds. These may occur, for example, on nickel-plated aluminum substrates used in the manufacture of thin-film magnetic media, but may pose a problem with respect to any smooth surface.
Surface pits are local depressions, and surface mounds are local protrusions of the surface with respect to its average level around the imperfection. They are but two classes of flaws that can be encountered. Others include defects, large and small of other morphology, including scratches, excessive surface roughness, digs, as well as contaminants of various natures including dirt, dust, oil, fingerprints, and the like. Defects on the surface of a rigid magnetic media may be either the result of an impingement onto the surface, or of a deposition of a foreign material on the surface, or of a tearing of material away from the surface, or of a combination of these phenomena.
Both the surface pits and surface mounds of interest have diameters of between a few micrometers to several hundred micrometers and depths (or heights) of a few hundredths to one micrometer typically. Both pits and mounds may be smooth or contain breaks or craters on their surface.
There are devices known which detect flaws generally, and which distinguish between pits and flaws caused by scratches and other flaws such as surface contamination including dirt or oil and the like. Reference is made to U.S. Pat. Nos. 4,794,264; 4,943,734; and 4,794,265, assigned to the same assignee as the instant application, and incorporated as if fully set forth herein by this reference.
Distinguishing between a pit and a mound, however, once such a flaw is detected, is useful in many applications. For example, a magnetic head used to write information on a magnetic disk could be damaged by a mound, whereas only a loss of information would result as the magnetic head passes over a pit. Technological advances have been made to the extent that the magnetic head quickly passes over the disk at an elevation of only a few microinches, (0.05-0.1 micrometers). Hence, a mound rising to an elevation higher than even a few hundredths of a micrometer could severely damage the head.
The equipment embodying the patents referenced above would automatically determine that a flaw exists generally, but the operator must still use a microscope to inspect the surface and determine whether an individual flaw is a pit or a mound, and also to determine, for a mound, whether it rises to an elevation which could cause damage to a magnetic head. Such inspection is time consuming, subject to error, and could result in further contamination due to the handling required. Moreover, such inspection is subjective in that it will not always be readily apparent whether a given mound rises in elevation to the point where magnetic head damage could occur. In short, such inspection is unreliable.